Using a 3/C with ground cable and bonding bushings on conduit

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turtlepokerman

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Greetings,

I'm going a little crazy here with the "equipment ground" versus a "bonding ground". What I currently have is a feeder cable that goes from a 70A - 480V feeder breaker to a 100A Disconnect switch. The cable is a 3/C #4 w/ gnd cable routed through aluminum emt conduit. The inspector on site says that I need bonding bushings on both ends of the conduit to maintain electrical continuity between the panels and the disconnect switch. The current conduit fittings are not UL listed as a grounding means.

Why would I need the conduit bushing on both ends of the raceway if I run a ground conductor in the cable? If a fault occurs in the raceway, the fault current would go through the equipment ground in the cable. Am I missing something?

Any help is appreciated.
 
What type of fittings is a good question. Being that it's a 480 volt circuit, i believe the inspector is probably looking at 250.97 - bonding for over 250 volts.

The way the holes are made in your enclosure, and the types of fittings you used will determine if your conduit is bonded. The actual conduit material is irrelevant for that code. It only references metal raceways, not what type of metal

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turtlepokerman said:
The current conduit fittings are not UL listed as a grounding means.
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Missed that last sentence. So i 2nd infinity, where did you get emt fittings that aren't listed? It doesn't sound like your conductor are too big that it would be difficult to add a bonding bushing.

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The plastic bushings are UL Listed, just not as a grounding means. The locknuts are listed as well. The concern is not the work required to add a bonding bushing the question is why is it required. I think 250.97 gave me a good start on the conversation. I guess it doesn't make sense then to ever pull a 3/C cable with ground if you are running a conduit system with bonds on both ends; otherwise, you are duplicating your grounding.

WarrMann said:
Missed that last sentence. So i 2nd infinity, where did you get emt fittings that aren't listed? It doesn't sound like your conductor are too big that it would be difficult to add a bonding bushing.

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turtlepokerman said:
I guess it doesn't make sense then to ever pull a 3/C cable with ground if you are running a conduit system with bonds on both ends
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That will likely come down to who you talk to. Some guys always run an egc cause they think its a more reliable and long term option than relying on conduit fittings to maintain a good ground. Some guys think a conduit is the better, more reliable, and obviously cheaper option.

I prefer to avoid pulling cables in a conduit, separate conductors is cheaper and easier to pull. I also prefer to run a ground wire

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Counter-question: How does the fault current get to the raceway if the energized source is inside the cable with the EGC? If I have a TC-ER cable that someone goes up and grabs versus a conduit with a tc-er cable what is the difference? Wouldn't they pose the same amount of risk?

electrofelon said:
but how does a fault current to the raceway get to the EGC in the cable?

In general you need bonding bushings around concentric/eccentric KO's (unless they are listed for grounding).
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turtlepokerman said:
Counter-question: How does the fault current get to the raceway if the energized source is inside the cable with the EGC?
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EGC should be bonded to the enclosure, just like the conduit should be. The fault current runs through the EGC, goes through the enclosure, then travels to the conduit via the enclosure.

That's a moot point in your situation though because the fault current will just take the EGC back to the source. It's unlikely it will need take a path on the conduit... because you have a lower resistance path on the conductor

In your case, you need to bond the conduit to prevent it from becoming and staying live. Your EGC will be the reason the breaker trips in the event of a short.

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WarrMann said:
That's a moot point in your situation though because the fault current will just take the EGC back to the source. It's unlikely it will need take a path on the conduit... because you have a lower resistance path on the conductor
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Why do you think this? Why wouldn't fault current take all paralleled paths to clear a fault?



Did the OP ever state what type of cable this is?
 
david said:
Why do you think this? Why wouldn't fault current take all paralleled paths to clear a fault?



Did the OP ever state what type of cable this is?
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Path of least resistance is also the shortest path. For example - we cannot just tap one set of conductors from a parallel service

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turtlepokerman said:
The plastic bushings are UL Listed, just not as a grounding means. The locknuts are listed as well. The concern is not the work required to add a bonding bushing the question is why is it required. I think 250.97 gave me a good start on the conversation. I guess it doesn't make sense then to ever pull a 3/C cable with ground if you are running a conduit system with bonds on both ends; otherwise, you are duplicating your grounding.
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Again others have asked where you got an EMT fitting that is not listed? If it is listed I'm pretty sure it will also meet grounding requirements. The bushing is a separate fitting and doesn't have grounding/bonding requirements, the EMT fitting in combination with the locknut will make a bond to a metallic enclosure - if you have concentric/eccentric knockouts or other situations of impaired continuity that is a separate issue that may trigger the need for a bonding bushing or other bonding methods, but as long as one end of that raceway is bonded properly the entire raceway is bonded and the other end can still have impaired continuity issues without the need to add more hardware to that end, when there is also an EGC ran within the raceway and it bonds to the EGC/enclosure on the other end.

turtlepokerman said:
Counter-question: How does the fault current get to the raceway if the energized source is inside the cable with the EGC? If I have a TC-ER cable that someone goes up and grabs versus a conduit with a tc-er cable what is the difference? Wouldn't they pose the same amount of risk?
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The metal raceway needs to be bonded even if there is a wire type EGC within it. Listed fitting and proper installation to a bonded metal enclosure will accomplish this. Impaired bonding at the fitting will require additional bonding means to be used. Bonding bushings are typically the most common option to do so but there are other ways.
 
The metal raceway needs to be bonded regardless of what's inside of it to insure a low impedance path back to the source.

You cant pick and choose were a short circuit might occur.

A short may occur inside the cable that's inside the conduit and never contact the raceway. In that instance, yes, the cable would use the EGC incorporated in the cable to clear the fault.

If the cable happened to short out to the conduit, the conduit would have to be bonded to provide that path back to the source.

There is not enough information in the first post to clearly picture your install. For all we know the cable could leave the panel in a conduit, up to a cable tray, out of the cable tray down to the piece of equipment it's feeding.

In that scenario, you could not solely rely upon the EGC located in the cable to clear a fault. The EMT raceways would need to be properly bonded also.


JAP>
 
turtlepokerman said:
The current conduit fittings are not UL listed as a grounding means.
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sgb75.jpg


turtlepokerman said:
Why would I need the conduit bushing on both ends of the raceway if I run a ground conductor in the cable? If a fault occurs in the raceway, the fault current would go through the equipment ground in the cable. Am I missing something?
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The plasma of the arc will make quick work of the cable material and likely expand to the raceway. Likely enough that the energy passing from the raceway connector to the enclosure will start burning open like a fuse, without additional bonding of the raceway to the enclosure.
 
al hildenbrand said:
sgb75.jpg



The plasma of the arc will make quick work of the cable material and likely expand to the raceway. Likely enough that the energy passing from the raceway connector to the enclosure will start burning open like a fuse, without additional bonding of the raceway to the enclosure.
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But what "EMT fittings" are out there that are not also rated for grounding/bonding?

Only conduit fittings that I recall having encountered that can't be used for grounding/bonding is FMC connectors with a single set screw - but only when used with aluminum flex.

I think OP and/or his inspector is hung up on his plastic bushing he used not being listed for grounding/bonding, but it doesn't need to be. Grounding bushings are an option when you need to improve a poor or non existent bond at the raceway fitting itself.
 
electrofelon said:
Careful. Current/fault current takes all paths. Nor does least length necessarily translate to least resistance.
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Correct, but a reason we install EGC's is to provide a low resistance path for facilitating such fault currents when they occur with the intent to direct the bulk of that fault through a prescribed path and to allow the overcurrent device to sense the fault quickly because of the high current so it can respond. Now if you are in a steel structure and have your wiring methods attached to the structure, the structure may very well have a lower resistance anyway and carry the bulk of any fault current.
 
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